One such thermal control unit known from British Patent Specification No. GB-A No. 1401954, is adapted to be secured to the head of an electric heating element of an immersion heater, such head itself being adapted to be mounted in or adjacent an aperture in the wall of a vessel to be fitted with such heating element. The control unit has terminals for electrical connection to the cold tails of the heating element and further incorporates a thermally-sensitive electric switch including a bimetallic snap-acting switch-actuating member positioned so as in use of the control unit to be in good thermal contact with the element head, the bimetallic switch-actuating member being of a type comprising a stressed piece of bimetallic material which moves with changes in temperature between two oppositely dished configurations with a snap action and being mounted so as in its cold condition to present its concave face to the heating element which is formed with a complementarily shaped projection engaging the bimetallic member in supposedly good thermal contact. As is well known, the principal purpose of the thermal control unit is to protect the heating element of the immersion heater by automatically cutting off or reducing the electric power supply to the heating element if it overheats, for example in the case of the vessel boiling dry or being switched on when empty.
While the thermal control unit of GB-A No. 1401954 has been widely and successfully marketed, nonetheless it has given rise to a number of problems. Firstly, it has been found difficult to consistently maintain the positional relationship between the bimetallic element and the complementarily shaped projection on the element head, inter alia because of manufacturing difficulties encountered with the element head and an encountered tendency for the element head to distort. As will be appreciated by those skilled in the art, the dimensional integrity of bimetallic elements of the snap-acting type disclosed in GB-A No. 1401954 is essential to the element demonstrating consistent switch-actuating operation within defined temperature limits, and any tendency of the element head to distort can, with the arrangement of GB-A No. 1401954, lead to bending of the central tongue of the bimetallic blade with consequent variation in the switching characteristics of the blade.
A further difficulty which has been encountered stems from the formation of the element head projection by a stamping process which leaves a corresponding depression on the heating element side of the head. To secure efficient thermal transfer between the element proper and the head, it then proved necessary to enter the heating element into this depression and to braze it to the head with silver solder which not only is expensive, but also has been found to tend to transmit the heat of the element excessively towards the periphery of the immersion heater head with consequent risk of overheating of adjacent plastic material as, for example, when the immersion heater is used with a plastic bodied vessel and/or is secured to a vessel by means of a plastic locking ring.
Other problems with the arrangement of GB-A No. 1401954 and with othre similar arrangements stem from the fact that, as the bimetallic blade begins to move towards its intermediate unstable condition prior to snapping to its alternative configuration, it moves out of thermal contact with the element head and thus a silicone oil based heatsink compound must to be employed, with attendant manufacturing and other disadvantages.
The abovementioned problems are also encountered in the element protection unit, which is the subject of British Patent Specification GB-B No. 2117568 (Otter Controls Ltd) and corresponding U.S. Pat. No. 4,539,468 and which pioneered the concept of providing two independently operating snap-acting bimetallic thermal sensors in such a unit, and with the two independent sensors in effect providing primary and secondary protection, the secondary protection providing a back-up in the event that the primary protection fails. Even in the arrangement disclosed in European Patent Specification No. EP-A No. 0202939 (Otter Controls Ltd), which utilizes two partially overlapped bimetallic blades which nest with a generally complementarily shaped double-dimple formation specially provided in the element head, the potential exists for problems to arise as regards locating the bimetals relative to the element head and in good thermal contact therewith when consideration is given to the relatively imprecise manufacturing tolerances of electric heating elements.
Another proposal for the provision of both primary and secondary or back-up protection is disclosed in British Patent Specification No. GB-A No. 2181598 (Strix Ltd). According to this proposal, a bimetal held in contact with the element head controls the operation of a primary pair of switching contacts in a conventional manner, and a contact of a secondary pair of switching contacts is supported by a fusible peg so that, in the event of failure of the bimetal so that the element continues to heat up, the fusible peg gives way and allows the secondary contacts to open. This proposal also suffers from the aforementioned disadvantages, and has the further disadvantage that the fusible peg may be prone to thermally induced creepage which, over a period of time, could lead to arcing between the secondary contacts and corresponding deterioration of the control. Other disadvantages arise in that fusing together of the contacts, for example on account of arcing, could totally disable the secondary protection afforded by the control in an unsafe condition.
Other prior art publications of interest are GB-A Nos. 1408387, 2133630, 2149217, EP-A No. 0014102, U.S. Pat. Nos. 4,433,231 and 4,295,114.